Sorry, I don't have any experience or even word of mouth info on the MSD unit. However, just from working with my system I might have some answers (right or wrong) to your questions. Here is the Atomic forum that I would spend some time on if I were you:
Is it worth the upgrade? (To get away from a quadrajet for a guy, like me, that is not that good with carbs. Ive had the carb rebuilt but it is still not perfectly tuned in so it would need a dyno tune if I don't. $$$)
Debatable. You'd likely never pay off the unit in fuel savings alone. However, it is nice to be able drive the car at any elevation or temperature and have it adjust on its own. You can get a good bump in "under the curve" power, and low speed is much improved over a carb if you have a decent cam....I can accelerate from 1000 RPM up in fifth with a 0.64 OD and 3.42 rear on a flat road and can cruise in fifth at 1300 RPM under just about any conditions. Having the ability to just tune problem areas with a laptop is nice too although I don't know that the MSD unit lets you do that. There are plenty of people who have a grand or better in their carb, tuning kits and parts, and time in getting them like they want them.
Can I use a singe fuel line? (The instructions suggest that most will need a return line, see attached, but I live in southern alberta canada and average high temperatures in the summer is 26*C or 79*F but does get up to 32*C or 90*F. I have a new tank already and dont want a return)
Don't see why not. The unit has a built in PWM controller for the pump but you must use the MSD pump. A return is easy enough to install later if it becomes an issue. Note that there is a warning in the instructions to NOT
use hard line. I don't know what type of hose they supply but if it's not Teflon then it will bleed fuel odor. It is worth it to spend the money on Teflon line and it can just be routed along the hard line. Down the line if you needed a return you could use the hard line for the return which is what I did with mine.
Do I need to upgrade to an MSD distributor and box? (currently I am running a Pertronix III setup so it will not be able to utilize the timing control function)
You only need to upgrade if you want to use the unit for timing control. If you have a good, well tuned setup I see no benefit to using their timing control other than tuning the timing from the handheld instead of playing with springs/weights/dizzy. Even then you cannot adjust rate of advance like you can with springs and adjustable vacuum cans. Retrotek (my unit) recently came out with a new version of their ECU with timing control and a couple other tweaks, and they have a dealer trade in program. It is more complex and feature laden than the MSD unit which is fairly basic (full timing table vs just start/end/vacuum timing inputs) and will work with most distributors but I couldn't see any benefit going to it given how well mine is set up now.
Where do you mount the fuel pump? (instruction states it is best to be below the lowest point of the tank and as close as possbile)
Here is mine. Just took a few pieces of angle iron, a little time with the welder, some old leaf spring pads, and some hardware. It is bolted to the fuel tank straps (button heads) and can stay in place if the tank has to come out. The button heads used to mount it to the straps are in positions where they do not touch the tank (right below and above the tank seam) and I used some strips of an old rubber floor mat between the bracket and straps.